Semiconductor interband lasers and method of forming

ABSTRACT

A semiconductor interband laser that includes a first cladding layer formed using a first high-doped semiconductor material having a first refractive index/permittivity and a second cladding layer formed using a second high-doped semiconductor material having a second refractive index/permittivity. The laser also includes a waveguide core having a waveguide core refractive index/permittivity, the waveguide core is positioned between the first and the second cladding layers. The waveguide core including an active region adapted to generate light based on interband transitions. The light being generated defines the lasing wavelength or the lasing frequency. The first refractive index and the second refractive index are lower than the waveguide core refractive index. The first cladding layer and/or the second cladding layers can also be formed using a metal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a divisional of the patent applicationidentified by U.S. Ser. No. 12/975,008, filed on Dec. 21, 2010 whichclaims benefit of U.S. Provisional Application No. 61/288,701, filedDec. 21, 2009, which is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract NumberDMR0520550 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor interband lasers and moreparticularly, to semiconductor mid-infrared diode lasers having improvedthermal dissipation and being cable of operation in longer wavelengthspectrum.

2. Brief Description of Related Art

Semiconductor lasers have been developed with emission wavelengths (λ)ranging from near- to mid-infrared (λ>3 μm) and beyond. When thewavelength of a semiconductor laser is long, the cladding layerthickness of the waveguide structure for the laser must be made thicker.For instance, mid-infrared interband cascade (IC) lasers typically use2-3 μm-thick InAs/AlSb superlattice (SL) as the cladding layer toconfine the optical wave in a waveguide. See, e.g., Yang, “Mid-InfraredInterband Cascade Lasers Based on Type-II Heterostructures”,Microelectronics J. Vol. 30, 1043 (1999); Hill, et al, “MBE GrowthOptimization of Sb-Based Interband Cascade Lasers”, J. Crystal Growthvol. 278, 167 (2005); Vurgaftman, et al, “Mid-infrared interband cascadelasers operating at ambient temperatures”, New J. Phys. Vol. 11, 125015(2009). The use of thick InAs/AlSb SL cladding layers in IC lasers isvery demanding for growth by molecular beam epitaxy (MBE) with so manyshutter movements. Furthermore, an InAs/AlSb SL layer has a very lowthermal conductivity (κ˜ 0.03 W/cm·K) as indicated by Borca-Tasciuc, etal. in the paper entitled “Thermal conductivity of InAs/AlSbsuperlattices” published in Microscale Thermophys. Eng. Vol. 5, 225(2001), and thick SL layers cause significant heating. Because the SLcladding layer has a refractive index (˜3.37) that is only slightlysmaller than that of the cascade region (3.43 to 3.47), its thicknesscannot be reduced, which could lead to substantial leaking of theoptical wave into the GaSb substrate (refractive index ˜3.8), resultingin undesirable optical loss. This situation will become worse if SLcladding layers are still used in IC lasers for longer wavelengthsbecause of the requirement of thicker cladding layers. Hence, if the SLcladding layer can be replaced with appropriate material, IC laserperformance will improve significantly. Also, it is feasible to extendefficient IC lasers to longer wavelengths.

Therefore, it is an object of the present disclsoure to provide animproved semiconductor mid-infrared diode laser with greater thermaldissipation and being adapted to generate or emit light having a lasingwavelength longer than lasing wavelengths using previous IC lasers.

SUMMARY OF THE INVENTION

The present invention relates to a semiconductor interband laser whereoptical transitions occur between the conduction band and the valenceband for photon emission. More particularly, but not by way oflimitation, the present disclosure relates to a plasmon waveguideinterband laser using a relatively high-doped semiconductor material(e.g. n+-type InAs, doped GaSb or other materials) or/and metal (e.g.Au) as the optical cladding layers to form a plasmon waveguide withoutusing thick SL layers (e.g. InAs/AlSb SL) or ternary (e.g. AlGaSb) orquaternary (e.g. AlGaAsSb) material layers.

In one aspect, the present disclosure relates to a semiconductorinterband laser. The laser has a first and second cladding layers formedusing a first and a second high-doped semiconductor materials. The firsthigh-doped semiconductor material has a first refractive index where thesecond high-doped semiconductor material has a second refractive index.The laser also includes a waveguide core having a waveguide corerefractive index. The waveguide core is positioned between the first andthe second cladding layers. The waveguide core includes an active regionadapted to generate light based on interband transitions. The lightbeing generated based on interband transitions defines the lasingwavelength (or lasing frequency) of the laser. The first refractiveindex and the second refractive index are lower than the waveguide corerefractive index.

In accordance with another aspect, disclosed is a semiconductorinterband laser. In this aspect, the laser includes a first claddinglayer formed using a high-doped semiconductor material having a firstpermittivity and a second cladding layer formed using a metal having asecond permittivity. The laser also includes a waveguide core having awaveguide core permittivity, the waveguide core is positioned betweenthe first and the second cladding layers. The waveguide core includes anactive region that is adapted to generate light based on interbandtransitions. The light being generated based on interband transitionsdefines the lasing wavelength (or lasing frequency) of the laser. Thefirst permittivity and second permittivity are lower than the waveguidecore permittivity.

In yet another aspect, disclosed is a semiconductor interband laser. Thelaser includes a first cladding layer formed using a metal materialhaving a first permittivity and a second cladding layer formed using ametal material having a second permittivity. The laser also includes awaveguide core having a waveguide core permittivity, the waveguide coreis positioned between the first and the second cladding layers. Thewaveguide core includes an active region that is adapted to generatelight based on interband transitions. The light being generated based oninterband transitions defines the lasing wavelength (or the lasingfrequency) of the laser. The first permittivity and second permittivityare lower than the waveguide core permittivity.

The term “relatively high-doped” or simply “high-doped” semiconductormaterial, as used herein, refers to a semiconductor material that has adoping concentration higher than the doping concentrations typicallyused. Doping of semiconductor materials generally refers to theintroduction of impurities into the crystalline structure to, forexample, change the conductivity and/or permittivity of thesemiconductor material, as is known in the art. As is also understood inthe art, the permittivity of a medium (e.g., a semiconductor material)is a function of the square of the refractive index. In one aspect, theuse of high-doped semiconductor material used to form the claddinglayers in the present disclosure is to lower the refractiveindex/permittivity of the semiconductor material to less than therefractive index/permittivity of an waveguide core, having an activeregion, of the laser at the lasing frequency/wavelength. In anotheraspect, the purpose of using high-doped semiconductor materials is toincrease the plasmon frequency of carriers in the semiconductor materialso that the plasmon frequency of the semiconductor material is closer tothe lasing frequency/wavelength of the laser. Previously, the lasingfrequency is usually much higher than the plasmon frequency. When thelasing frequency is close to the plasmon frequency, the refractiveindex/permittivity of the material at the lasing frequency is lowered.As such, the high-doped semiconductor material can be used as a goodoptical cladding layer. Exemplary values of “relatively high-dopedsemiconductor material” is for material with doping concentrations in arange approximately from 10¹⁸ cm⁻³ to 10¹⁹ cm⁻³. In accordance withanother aspect of the present disclosure, additional exemplary valuesinclude doping concentrations in the range approximately from 10¹⁸ cm⁻³to 10²⁰ cm⁻³

It should be understood that the level of doping will depend upon thetype of semiconductor material utilized as the plasmon waveguide and canvary from the specific examples provided in this document. The claddinglayers are used to confine the optical wave mainly in the active region,where the permittivity in the cladding layers is smaller than that inthe active region. The active region with the higher permittivity andthe cladding layers (with the lower permittivity) form a waveguidestructure, where the wave is mainly propagating inside the activeregion. The cladding layers can be constructed of different materials,and the cladding layers can have the same level of doping, or differentlevels of doping.

Additionally, references herein to the plasmon frequency being “close”or “closer” to the lasing frequency are understood to refer to plasmonfrequencies of the semiconductor materials used to form the claddinglayers. For example, for an interband cascade laser where the refractiveindex of the active region is 3.42, and the emission wavelength is 6 μm(ω=1660 cm⁻¹), it is desirable to adjust the doping so that the plasmafrequency of the cladding layers is 500 cm⁻¹ or higher. This makes therefractive index of the cladding layers less than 3, ensuring good modeconfinement.

In addition, it should be understood that although an embodiment of theplasmon waveguide interband semiconductor laser described herein isbased on an InAs substrate, the idea of using a plasmon waveguide as acladding material for a semiconductor laser is more general and can beimplemented with other materials and substrates. For example, theplasmon waveguide interband cascade laser can be implemented based on aGaSb substrate with doped GaSb materials.

It should be understood that InAs has a thermal conductivityapproximately 10 times higher than that of the InAs/AlSb SL. Metals thatmay be used for cladding such as Au and Ag have thermal conductivitiesthat are about 100 times higher than the InAs/AlSb SL. Thus, thereplacement of the SL cladding layers with doped InAs or metal layerssignificantly improves heat dissipation in the laser (e.g. interbandcascade lasers). Also, the growth of plasmon waveguide IC laserstructures by molecular beam epitaxy (MBE) without thick SL layers wouldbe much less demanding with dramatically reduced shutter movements onmany (>1000) interfaces of the SL. Other benefits with the plasmonwaveguide for IC lasers are elaborated below.

The square of plasmon frequency ω_(p) ² (=ne²/m*∈_(∞)∈₀) of a materialis proportional to the electron concentration n, and inverselyproportional to the electron effective mass m* and the high-frequencydielectric constant ∈_(∞). In n⁺-type InAs where electron effective mass(m*=0.023m₀, which will increase to some degree due to non-parabolicdispersion, where m₀ is the free electron mass) is small and∈_(∞)=12.25, the plasmon frequency ω_(p) for InAs even with not veryhigh doping concentration (<2×10¹⁹/cm³) can be comparable to the laserfrequency in mid-infrared spectrum. As such, the refractive index ofrelatively high-doped n⁺-type InAs can be less than 3.0, which issubstantially lower than that for the cascade region (˜3.43-3.47) in IClasers. Hence, with the use of InAs plasmon layers in the claddingregions and undoped (or lightly doped) n-type InAs (refractive indexnear 3.5) as the separate confinement layers (SCLs), the light is moreconfined to the center of the waveguide and extends less into thecladding layers and substrate, as shown in FIGS. 1 a and 1 b, showing anexample of a 10-stage two-sided plasmon-waveguide IC laser at 4.6 μm.Compared to the 20-stage regular (without SCLs) SL-cladding waveguide(FIG. 1 a) and the 10-stage SL-cladding SCL (with two GaSb SCLs)waveguide (FIG. 1 b), the optical mode penetration (Γ_(clad)) into thecladding regions is reduced for the plasmon waveguide where it is onlyabout 4%. In this way, optical loss will be suppressed since only a verysmall portion of the optical wave is in the relatively highly-dopedn⁺-InAs cladding regions where the loss may be significantly higher.This is supported by observations of low optical losses (e.g. 6-8 cm⁻¹)in InAs-based quantum cascade (QC) lasers at 4.5 and 10 μm with similarplasmon-waveguide structures as described by Teissier, et al. in “Roomtemperature operation of InAs/AlSb quantum cascade lasers”, Appl. Phys.Lett. Vol. 85, 167 (2004); and by Ohtani, et al. in “Mid-infraredInAs/AlGaSb superlattice quantum-cascade lasers”, Appl. Phys. Lett. Vol.87, 211113 (2005). In intersubband QC lasers, light emission is based ontransitions between the subband states within the same band (e.g.conduction band), while in interband lasers disclosed here, lightemission is based on transitions between the conduction and valencebands.

In metals, the plasma frequency falls in the ultraviolet band, and thepermittivity is negative. The optical confinement in the active regionwill be greatly enhanced in waveguides which use metal cladding incomparison to those using semiconductor material. Also, although theoptical absorption is higher in metals, it can be projected that withproper design, a low-loss waveguide can be designed due to the smallpenetration (<1%) into the cladding layer. Metal was used as opticalcladding layer in intersubband QC lasers as described by Sirtori et atin Optics Letters vol. 23, 1366 and by Tredicucci et al in U.S. Pat. No.7,382,806, while lasers described herein with metal cladding layers aredifferent because they are based on interband transition.

Previous IC lasers with InAs/AlSb SL cladding layers have higher opticalloss. For example, 11-17 cm⁻¹ at ˜78 K and 28-35 cm⁻¹ at hightemperatures (≧270 K), as described by Soibel, et al. in “Optical gain,loss and transparency current in high performance mid-IR interbandcascade lasers”, J. Appl. Phys. Vol. 101, 093104 (2007); and by Bewley,et al in “Gain, loss, and internal efficiency in interband cascadelasers emitting at λ=3.6-4.1 μm”, J. Appl. Phys, vol. 103, article013114 (2008). One possible reason for this high optical loss is thatthe SL cladding layer has a refractive index (˜3.37) that is justslightly lower than the value in the cascade core region, so the laserlight significantly extends into the cladding region as shown in FIG. 1(57% and 43% for regular and SCL waveguides, respectively). The opticalpenetration into the SL cladding layer may even extend into the highindex GaSb substrate with more optical loss when the bottom SL claddingregion is not sufficiently thick (for avoiding much worse thermaldissipation). Hence, compared to IC lasers on GaSb substrates, theoptical loss can be reduced in plasmon-waveguide IC lasers with muchless wave penetration into the cladding regions. In addition, theplasmon-waveguide IC lasers can be grown on n⁺-type (1-3×10¹⁸ cm⁻³available commercially) InAs substrates, which have a permittivity lowerthan that of the waveguide core region and can be used as an extracladding layer. Also, the operating voltage can be reduced further forplasmon waveguide SCH IC lasers with fewer number of cascading stages.Consequently, the power consumption in plasmon waveguide IC lasers wouldbe reduced significantly.

Therefore, plasmon waveguide IC lasers on InAs substrates can havebetter performance and efficient continuous wave (cw) operation withsignificantly improved thermal dissipation, reduced optical loss andpower consumption. Another important feature is that plasmon waveguideIC lasers are believed to be able to achieve efficient cw operation inthe longer wavelength (>6 μm) region without the difficulties associatedwith thick SL cladding layers. Electroluminescence from IC lightemitting diodes (without cladding layers) has been demonstrated in 6-15μm wavelength region. However, it is difficult for current IC laserswith the thick SL cladding layers to cover the longer wavelength regionbecause the optical confinement for such a long wavelength laser lightrequires much thicker SL cladding layers, which not only makes thegrowth more challenging but also leads to much worse thermaldissipation. Hence, by circumventing thick SL cladding layers, plasmonwaveguide IC lasers are capable of efficient cw operation with low powerconsumption in a wide range of wavelength spectrum including traditionaldifficult long wavelength (>6 μm) IR region for III-V interband diodelasers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

So that the above recited features and advantages of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference to theembodiments thereof that are illustrated in the appended drawings. It isto be noted, however, that the appended drawings illustrate only typicalembodiments of this invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIGS. 1( a) and 1(b) are line graphs of the simulated optical modal andrefractive index profiles for a 10-stage plasmon-waveguide vs. (a)20-stage regular, and (b) 10-stage SL-SCL IC laser structures, inaccordance with the present disclosure.

FIG. 2 is a schematic diagram of a semiconductor interband laserconstructed in accordance with the present disclosure.

FIG. 3 is a schematic diagram of a more particular embodiment of aninterband laser constructed in accordance with the present disclosure.

FIG. 4 is a schematic diagram of an interband cascade laser constructedin accordance with the present disclosure.

FIG. 5 is a graph illustrating laser spectra (normalized as indicated)of a 150-μm-wide mesa-stripe laser (device 2H, just above threshold) incw mode and at several temperatures, constructed in accordance with thelaser shown in FIG. 4.

FIG. 6 is a graph showing the current-voltage-light (I-V-L)characteristics of the device 2H in cw mode at several temperatures, thetemperature for the I-V curves being indicated by the label on thecurve, in accordance with one aspect of the present disclosure.

FIG. 7 is a graph showing the I-V-L characteristics of a narrow ridgelaser, constructed in accordance with the laser shown in FIG. 4, in cwmode at various temperatures, the heat-sink temperature for the I-Lcurves being labeled beside the curves, showing the laser spectrum at184K in the inset.

FIG. 8 is a graph showing the I-V-L characteristics of a narrow ridgelaser operating in cw mode at various temperatures, the laser spectrumat 141 K being shown in the inset, in accordance with the presentdisclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The Presently preferred embodiments of the invention are shown in theabove-identified figures and described in detail below. In describingthe preferred embodiments, like or identical reference numerals are usedto identify common or similar elements. The figures are not necessarilyto scale and certain features and certain views of the figures may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

Referring now to the drawings, and in particular to FIG. 2, showntherein and designated by reference numeral 10 is a schematic diagram ofa semiconductor interband laser constructed in accordance with thepresent disclosure. The laser 10 includes a first cladding layer 12, asecond cladding layer 14, and a waveguide core 16 positioned between thefirst and second cladding layers 12 and 14. The waveguide core 16further includes an active region 18.

In a first embodiment of the laser 10, the first cladding layer 12 isformed using a first high-doped semiconductor material having a firstrefractive index, whereas the second cladding layer 14 is formed using asecond high-doped semiconductor material having a second refractiveindex. In this embodiment, the first and second high-doped semiconductormaterials can be the same materials, or they can be formed usingdifferent materials. That is, the first high-doped semiconductormaterial can be a different material than the second high-dopedsemiconductor material. The waveguide core 16, including the activeregion 18, also has a waveguide core refractive index. In a preferredembodiment of the present disclosure, the first and second refractiveindexes are lower than the waveguide core refractive index.

The first and second cladding layers 12 and 14 are formed withhigh-doped semiconductor material having doping concentrations in arange of between 10¹⁸ cm⁻³ to 10¹⁹ cm⁻³. In another aspect, the dopingconcentrations can be in a range of between 10¹⁸ cm⁻³ to 10²⁰ cm⁻³. Inanother aspect, the high-doped semiconductor material used to form thefirst and second cladding layers 12 and 14 is an n⁺-type InAs with adoping concentration of at least 10¹⁸ cm⁻³.

In a second embodiment, the first cladding layer 12 is formed using ahigh-doped semiconductor material having a first permittivity, whereasthe second cladding layer 14 is formed using a metal having a secondpermittivity. In this second embodiment, the waveguide core 16, havingthe active region 18, also has a waveguide core permittivity. In thissecond embodiment, the first and second permittivity are lower than thewaveguide core permittivity. As discussed above, refractive index is afunction of permittivity, and vice-versa. Exemplary metals used to formthe second cladding layer 14 include, but are not limited to, Ag, Au,Cu, Ti, Pt, Ni, and Pd, or combinations thereof.

In a third embodiment, the first cladding layer 12 is formed using afirst metal having a first permittivity, whereas the second claddinglayer 14 is formed using a second metal having a second permittivity.Similarly, the waveguide core 16, having the active region 18, has awaveguide core permittivity. In this embodiment, the first and secondpermittivity is lower than the waveguide core permittivity. Exemplarymetals used to form the first and/or the second cladding layers 12 and14 include, but are not limited to, Ag, Au, Cu, Ti, Pt, Ni, and Pd, orcombinations thereof.

The active region 16 is adapted to generate light based on interbandtransitions. As would be understood in the art, the light beinggenerated based on interband transitions thereby defines the lasingwavelength, or the lasing frequency of the laser 10. A more particularexample, of an active region 16 is described herein below with respectto the following figures. In one aspect, the active region 16 caninclude an interband cascade region. However, other active regions arealso considered within the scope of the present disclosure. Inaccordance with another aspect of the present disclosure, the activeregion 16 is adapted to generate light based on interband transitionswherein the wavelength of the light is greater than 3.0 μm.

In another aspect of the presently described embodiments, the waveguidecore 16 further includes one or more separate confinement regions (SCR)positioned between the active region 16 and first cladding layer 12, andthe active region 16 and the second cladding layer 14. For example, asshown in FIG. 2, the waveguide core 16 includes a first separateconfinement region 20 positioned between and separating the activeregion 16 from the first cladding layer 12 and a second separateconfinement region 22 positioned between and separating the activeregion 16 from the second cladding layer 14. Although the first andsecond separate confinement regions 20 and 22 are shown in FIG. 2 asbeing a single layer, it is to be understood that the regions 20 and 22can be formed using a plurality of layers of materials. Exemplarymaterials that can be used to form the regions 20 and 22 include, butare not limited to, InAs, InAsSb, InGaAs, InGaAsSb, GaSb, GaInSb,AlGaSb, AlGaInSb, GaAs, AlInSb, AlSbAs, AlGaSbAs, or AlGaInSbAs, orcombinations thereof. As discussed above, the materials forming thelayers of the regions 20 and/or 22 can be doped, low-doped, or notdoped.

With reference now to FIG. 3, shown therein is schematic diagram of amore particular embodiment of the laser 10 constructed in accordancewith the present disclosure. This embodiment of the laser 10 includesthe first and second cladding layers 12 and 14, and the waveguide core16 positioned between the cladding layers 12 and 14, the active region18 separated from the cladding layers 12 and 14 by the first and secondseparate confinement regions 20 and 22, respectively. In thisembodiment, the laser 10 is grown on a substrate 24 formed of, forexample, InAs material having a substrate refractive index orpermittivity. In one aspect the refractive index or permittivity of thesubstrate 24 is lower than the waveguide core refractive index or thewaveguide core permittivity.

In another aspect, disclosed is a method of forming a semiconductorinterband laser 10. The first cladding layer 12 is grown, for example,on a substrate 24 that can be InAs or other semiconductor material. Thefirst cladding layer 12 can be formed using a high-doped semiconductormaterial. The material used to form the first cladding layer 12 has afirst refractive index/permittivity. A waveguide core 16 is grown on thefirst cladding layer 12. The waveguide core 16 includes an active region18 that is adapted to generate light based on interband transitions. Thelight defines the lasing wavelength or the lasing frequency of the laser10. The waveguide core 16 has a waveguide core refractiveindex/permittivity. A second cladding layer 14 is grown on the waveguidecore 16. The second cladding layer 14 is formed using a secondhigh-doped semiconductor material or a metal. The material used to formthe second cladding layer 14 has a second refractive index/permittivity.The first and second refractive indexes/permittivities are lower thanthe waveguide core refractive index/permittivity.

With reference now to FIG. 4, shown therein is a schematic diagram of aspecific example of an IC laser 100 constructed in accordance with thepresent disclosure. The IC laser 100 shown in FIG. 4 was grown in a GenII molecular beam epitaxy (MBE) system on an n-type InAs substrate 110,with the exception of a top metal layer 112 that was deposited in athermal evaporator after MBE growth. The laser 100 comprises aninterband cascade region 114 having a thickness of 495 nm. In theembodiment shown in FIG. 4, the cascade region 114 consists of 10alternating active regions 120 and injection regions 122. The activeregion 120 and the injection region 122 form an interband cascade stagethat has a thickness of 495 Å. A cascade stage comprises many layersthat are made of compound semiconductor materials InAs, Ga(In)Sb, AlSbwith AlAs interfaces for balancing compressive strain from AlSb andGa(In)Sb layers. For the example shown in FIG. 4, the thickness of eachlayer is specified and was designed for lasing at ˜5.5 μm at atemperature of ˜80K. These 10 cascade stage region 114 is sandwiched bySCL regions 130 and 132 that are made of undoped or slightly doped InAs.In the embodiment shown in FIG. 4, the cascade region 114 and the SCLregions 130 and 132 form the waveguide core 16 discussed above that issandwiched by the first and second cladding layers 12 and 14. In thisembodiment, the first and second cladding layers 12 and 14 are formedusing two high doped n-type InAs materials that were heavily doped withSi up to 6×10¹⁸ cm⁻³ in this example. The InAs first cladding layer 12is 1.5-μm thick and serves as the bottom optical cladding layer in theexample shown in FIG. 4. The top InAs second cladding layer 14 is only35-nm thick and thus is too thin to confine an optical wave at amid-infrared wavelength. In this embodiment, the top InAs secondcladding layer 14 is used as a metal contact layer. Above the secondcladding layer 14, a metal layer 112 is deposited with a typicalthickness of 200 to 300 nm, which serves as the top optical claddinglayer to confine the optical wave in the waveguide.

Semiconductor laser devices can be manufactured by any known processes.The IC laser structure shown in FIG. 4 was processed into deep-etched150-μm-wide mesa-stripe and narrow (˜15 or 20 μm-wide) ridge laserdevices, both with metal contacts on the top layer and bottom substrate.Laser bars were cleaved to form 1.5-mm to 2.0-mm-long cavities, withboth facets left uncoated. The laser bars were affixed with indium,epilayer side up, onto a copper heat-sink and then mounted onto thetemperature-controlled cold finger of an optical cryostat for testing attemperatures of ≧82K.

With reference now to FIG. 5, shown therein is a spectra showing cwlasing spectra of a 150-μm-wide by 1.9-mm-long mesa-stripe laser(denoted as device 2H) at temperatures from 82 K to 150 K with lasingwavelength from 5.6 to 5.9 μm in good agreement with the design. Thecurrent-voltage-light (I-V-L) characteristics are shown in FIG. 6. Theoutput power of this laser exceeded 80 mW/facet at 82 K with a thresholdcurrent density of ˜49 A/cm², which is the lowest ever reported fordiode lasers operating in the wavelength region beyond 5.5 μm. Thethreshold voltage was ˜2.38 V at 82 K, which is only about 0.17 V higherthan the minimal required bias voltage for the 10-stage IC laser. Thisdemonstrates the efficient use of bias voltage in Plasmon waveguide IClasers. The specific thermal resistance for the broad-area device wasestimated to be ˜23 Kcm²/kW at 140 K. This value is lower than thetypical specific thermal resistance for broad-area IC lasers, whichsuggests an improvement in thermal dissipation for InAs plasmonwaveguide IC lasers 10 and 100 over IC lasers using InAs/AlSb SLcladding layers.

FIG. 7 shows current-voltage-light (I-V-L) characteristics of anarrow-ridge (15 μm) device with a cavity length of 1.80 mm. It lased upto 184 K at 5.9 μm in cw mode (see inset FIG. 7), with a thresholdcurrent density of 52 A/cm² at 80 K. The output power of this laserexceeds 40 mW/facet, with a slope efficiency of ˜100 mW/A per facet.There is no sign of saturation in the I-L curve with an injectioncurrent density of ˜2 kA/cm² at 80 K.

In summary, a new type of IC laser 10 and 100 has been demonstrated onan InAs substrate with cw operation up to 184 K near 6 μm. Thisrepresents a significant advance in the development of III-V mid-IRinterband diode lasers. By adjusting layer thickness of the activeregion 120 and injection region 122, longer wavelength IC lasers can beachieved. Such wavelength flexibility is illustrated in FIG. 8 showingcurrent-voltage-light (I-V-L) characteristics of a narrow-ridge (20 μm)device with a cavity length of 1.66 mm. It lased up to 141 K near 7.2 μmin cw mode (see inset FIG. 8).

In one aspect of the present disclosure, the example shown in FIG. 4 anddescribed above is for an IC laser 10 and/or 100 that uses one highlydoped semiconductor plasmon layer and one metal layer as opticalcladding layers. It should be understood that two highly dopedsemiconductor Plasmon layers with adequate layer thickness (>1 μm) canbe used as the first and the second cladding layers 12 and 14,respectively, to form a two-sided semiconductor Plasmon waveguide. Also,the substrate can be even removed by known techniques so that a metallayer can be deposited directly adjacent to the bottom n⁺-type InAsfirst cladding layer 12 to form a double-metal waveguide, in which layer12 can be very thin (e.g. tens-hundreds of nm) because the metal layercan be used as the bottom optical cladding layer.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred and alternativeembodiments of the present invention without departing from its truespirit. For example, the active region 120 and an injection region 122of the IC laser 100 can be constructed in a variety of manners and withvarious materials, and thicknesses of materials/layers. This descriptionis intended for purposes of illustration only and should not beconstrued in a limiting sense. The scope of this invention should bedetermined only by the language of the claims that follow. The term“comprising” within the claims is intended to mean “including at least”such that the recited listing of elements in a claim are an open group.“A,” “an” and other singular terms are intended to include the pluralforms thereof unless specifically excluded.

REFERENCES

-   U.S. PATENT DOCUMENTS: U.S. Pat. Nos. 6,301,282; 5,588,015;    5,799,026; 5,502,787; and 7,382,806

OTHER PUBLICATIONS

-   J. Z. Tian, R. Q. Yang, T. D. Mishima, M. B. Santos, and M. B.    Johnson, “Plasmon-Waveguide Interband Cascade Lasers Near 7.5 mm”,    Photonics Technol. Lett. 21, 1588 (2009).-   Rui Q. Yang, “Infrared Laser based on Intersubband Transitions in    Quantum Wells”, Superlattices and Microstructures, vol. 17 (1), pp.    77-83, 1995.-   R. Q. Yang, “Mid-Infrared Interband Cascade Lasers Based on Type-II    Heterostructures” Microelectronics Journal, vol. 30 (10), pp.    1043-1056, 1999.-   Ruin Q. Yang, “Novel Concepts and Structures for Infrared Lasers”,    Chap. 2, in Long Wavelength Infrared Emitters Based on Quantum Wells    and Superlattices, edited by M. Helm (Gordon & Breach Pub.,    Singapore, 2000).-   C. Sirtori et al., Long wavelength . . . , Appl. Phys. Lett., vol.    69, No. 19, pp. 2810-2812 (1996).-   Mansour, K., Qiu, Y., Hill, C. J., Soibel, A., and Yang, R. Q.:    ‘Mid-IR interband cascade lasers at thermoelectric cooler    temperatures’, Electron. Lett., 2006, 42 (18), pp. 1034-1035.-   Z. Tain, R. Q. Yang, T. D. Mishima, M. B. Santos, R. T.    Hinkey, M. E. Curtis, M. B. Johnson, “InAs-based interband cascade    lasers near 6 mm”, Electronics Letters, 45, 48 (2009).-   Yang, R. Q., Hill, C. J., Mansour, K., Qiu, Y., Soibel, A., Muller,    R., and Echternach, P.: ‘Distributed feedback mid-infrared interband    cascade lasers at thermoelectric cooler temperatures’, IEEE J.    Selected Topics of Quantum Electronics, 2007, 13, pp. 1074-1078.-   Vurgaftman, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, J. R.    Lindle, J. Abell, and J. R. Meyer, “Mid-infrared interband cascade    lasers operating at ambient temperatures”, New J. Phys. 11 125015    (2009)-   Kim, M., Canedy, C. L., Bewley, W. W., Kim, C. S., Linda, J. R.,    Abell, J., Vurgaftman, I., and Meyer, J. R.: ‘Interband cascade    laser emitting at λ=3.75 μm in continuous wave above room    temperature’, Appl. Phys. Lett., 2008, 92, p. 191110.-   Hill, C. J., and Yang, R. Q.: ‘MBE Growth Optimization of Sb-Based    Interband Cascade Lasers’, J. Crystal Growth, 2005, 278, pp.    167-172.-   Soibel, A., Mansour, K., Qiu, Y., Hill, C. J., and Yang, R. Q.:    ‘Optical gain, loss and transparency current in high performance    mid-IR interband cascade lasers’, J. Appl. Phys., 2007, 101, p.    093104.-   Ohtani, K., and Ohno, H.: “An InAs-Based Intersubband Quantum    Cascade Laser”, Jpn. J. Appl. Phys., 2002, 41, p. L1279.-   Sirtori, C., Gmachl, C., Capasso, F., Faist, J., Sivco, D. L.,    Hutchinson, A. L., and Cho, A. Y.: “Long-wavelength (λ≈8-11.5 μm)    semiconductor lasers with waveguides based on surface plasmons”,    Optics Letters, 1998, 23, p. 1366-1368.-   Teissier, R., Barate, D., Vicet, A., Yarekha, D. A., Alibert, C.,    Baranov, A. N., Marcadet, X., Garcia, M., and Sirtori, C.:    “InAs/AlSb quantum cascade lasers operating at 6.7 μm”, Electron.    Lett., 2003, 39, pp. 1252-1253.

What is claimed is:
 1. A semiconductor interband laser comprising: afirst cladding layer formed using a metal material having a firstpermittivity; a second cladding layer formed using a metal materialhaving a second permittivity; and a waveguide core having a waveguidecore permittivity and being positioned between the first and the secondcladding layers, the waveguide core including an active region togenerate light based on interband transitions, the light defining alasing wavelength; wherein the first permittivity and secondpermittivity are lower than the waveguide core permittivity.
 2. Thesemiconductor interband laser of claim 1, wherein the waveguide corefurther includes one or more separate confinement regions positionedbetween the active region and first cladding layer and the active regionand the second cladding layer, the one or more separate confinementregions including one or more layers of semiconductor material having apermittivity higher than the first and second permittivity.
 3. Thesemiconductor interband laser of claim 2, wherein the semiconductormaterial forming the one or more separate confinement regions isselected from the group consisting of InAs, InAsSb, InGaAs, InGaAsSb,GaSb, GaInSb, AlGaSb, AlGaInSb, GaAs, AlInSb, AlSbAs, AlGaSbAs, andAlGaInSbAs.
 4. The semiconductor interband laser of claim 1, wherein theactive region is comprised of an interband cascade region.
 5. Thesemiconductor interband laser of claim 1, wherein the active region ofthe waveguide core includes one or more semiconductor layers selectedfrom the group consisting of InAs, InAsSb, InGaAs, InGaAsSb, GaSb,GaInSb, AlGaSb, AlGaInSb, GaAs, AlSb, AlAs, AlInSb, AlSbAs, AlGaSbAs,and AlGaInSbAs.
 6. The semiconductor interband laser of claim 1, whereinthe lasing wavelength is greater than 3.0 μm.
 7. The semiconductorinterband laser of claim 1, wherein the metal material forming the firstand second cladding layer is selected from the group consisting of Ag,Au, Cu, Ti, Pt, Ni, and Pd.
 8. The semiconductor interband laser ofclaim 1, wherein the laser is grown on a substrate material selectedfrom the group consisting of InAs and GaSb.